There exists a need for the fabrication of ceramic materials having improved electronic properties which may be adjusted for a particular intended use. The present invention pertains to novel ceramic ferroelectric composite materials for use, for example, in low loss dielectric and ferroelectric applications such as wave-guides in phased array antennas and dielectrics in capacitors.
The ferroelectric materials are a replacement for the more expensive current driven ferrites that are currently used in phased array antennas. The present invention describes ferroelectric materials which provide adequate phase shift and have improved material properties which can be tailored for various applications. These properties include: (a) lower dielectric constants; (b) substantially decreased electronic loss, i.e., low loss tangents (tan .delta.); (c) increased tunability; (d) increased temperature stability; (e) decreased sintering temperature during manufacture; and (f) low Curie temperatures.
Current attempts to use ferroelectric materials employ porous ceramics whose properties are less than ideal for their intended applications. Porous ceramics of the barium strontium titanate type, Ba.sub.1-x Sr.sub.x TiO.sub.3, are commonly employed in ceramic phase shifter antennas. However, these materials display certain deficiencies due to manufacturing process difficulties and expense, as well as in their overall electronic and microwave properties. These deficiencies include electronic inhomogeneity, structural weakness, difficult reproducibility and process control during manufacture, and large loss tangents (tan .delta.).
Barium strontium titanate, Ba.sub.1-x Sr.sub.x TiO.sub.3 (BaTiO.sub.3 --SrTiO.sub.3), also referred to herein as BSTO, has been known to be used for its high dielectric constant, ranging from approximately 200 to 6,000, in various antenna applications. This was set forth by Richard W. Babbitt et al. in their publication, "Planar Microwave Electro-Optic Phase Shifters," Microwave Journal, Volume 35(6), June 1992. This publication concluded that more research needs to be conducted in the materials art to produce materials having more desirable electronic properties.
To address this need, it was subsequently discovered that BSTO could be combined with various metal oxides to produce ferroelectric composites having different and improved properties for particular applications. See, for example; U.S. Pat. No. 5,312,790 describing BSTO-alumina; U.S. Pat. No. 5,486,491 describing BSTO-zirconia; U.S. Pat. No. 5,635,433 describing BSTO-ZnO; U.S. Pat. No. 5,635,434 describing BSTO-magnesium based compounds incorporated by reference herein; and U.S. Pat. No. 5,427,988 describing BSTO-MgO composites and incorporated by reference herein. Of these, the BSTO-MgO composite has proven particularly important in that it possesses superior electronic properties for use in antenna systems.
The present invention provides a new class of ceramic ferroelectric composite materials with enhanced electronic properties comprising Ba.sub.1-x Sr.sub.x TiO.sub.3 --Mg and O containing compound composite, said composite being additionally doped with rare earth (lanthanide) oxides. The magnesium and oxygen containing compound is preferably MgO, but can also be selected from the group consisting of MgZrO.sub.3, MgZrSrTiO.sub.3, MgTiO.sub.3, and MgCO.sub.3. The doping can be carried out either with individual rare earth oxides or in combinations thereof. The rare earth oxides encompassed in the present invention include oxides of scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymuim (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). Where the rare earth element is represented by M, the oxides are generally of the formula M.sub.2 O.sub.3, although cerium gives cerium oxide CeO.sub.2. Moreover, it is intended that the rare earth oxide additives of the present invention include all oxidation states of the rare earth elements.